In reciprocating piston and internal combustion engines, connecting rods link the pistons to the crankshaft. Each connecting rod has a smaller end that is attached to a wrist pin in the piston and a larger end that is attached to the crankshaft. When the engine is run at high speeds, the connecting rods are subject to intense cyclic stress. Because the application demands that the connecting rods have great strength and hardness, these connecting rods are often made from steel or a steel alloy. Additionally, the application requires a precision connection between the ends of the connecting rod and the wrist pin and crankshaft to ensure smooth operation of the engine. Thus, the inner diameters of the ends of the connecting rod must be precisely dimensioned.
This combination of material and mechanical requirements has made powder metallurgy and, in particular, powder forging, a popular fabrication method for connecting rods over more traditional fabrication methods such as casting. Cast parts do not provide sufficient dimensional control of the connecting rod features. Additionally, cast parts require significant machining of the strong and hard material of the connecting rod. The lack of dimensional control means there is a substantial volume of difficult to machine material that needs to be removed. This machining is costly to perform and results in the production of a large amount of scrap material. However, connecting rods made using powder metallurgy have considerably more dimensional precision than cast parts. This increased amount of dimensional control reduces the amount of material that must be removed during machining and reduces the cost of secondary machining operations.
Preparation of a connecting rod using powder metallurgy is a multiple step process. First, a powder metal, which is often mixed with lubricant and wax, is pressed in a tool and die set to form a “green” compact. This green compact is then sintered in a furnace at temperatures just below the melting point of the main powder metal constituent. This sintering is typically performed in a reducing atmosphere to prevent oxidation of the connecting rod at the high temperatures. Although the sintered connecting rod is much stronger than the green compact, the sintered connecting rod is not fully dense. Because the connecting rod is not fully dense, the strength of the connecting rod is compromised. To further increase the density, the connecting rod can be powder forged. During powder forging, the sintered connecting rod is inserted into a die and subjected to stress at high temperatures. The application of stress at high temperatures induces plastic flow in the material, removing the excess porosity and results in a connecting rod that is nearly fully dense.
However, when made of steel or a steel alloy, even this nearly fully dense as-forged connecting rod is not considered suitable for use in an engine. When heated above certain temperatures, steel with certain amounts of carbon can form an austenitic phase in the microstructure. Upon cooling, this austenitic phase can form a martensitic phase. The amount of martensite and other phases formed is determined by the cooling profile of the part and can be approximated using a temperature, time, transformation diagram (TTT diagram) for the material being cooled. The martensitic phase is very hard, but is also very brittle. Due to this brittleness, the connecting rod would be incapable of sustaining the cyclic stresses applied during its use in an engine. Moreover, because the martensitic phase is hard and brittle, the machinability of the connecting rod is also reduced. To reduce this brittleness, the connecting rod must be tempered at temperatures lower than the solutionizing temperature to partially transform the martensitic phase into pearlite and bainite via carbon diffusion. This phase transformation toughens the steel and increases its ductility making the connecting rod suitable for use in an engine. The tempering step is often time consuming and can require substantial expenditures to provide the energy necessary to achieve the desired microstructure and its corresponding materials properties.
In addition to process variations, some of the materials properties of the connecting rod can be altered by the addition of alloying elements. Some alloying elements may directly alter the materials properties of the steel, while others may prevent the formation of undesirable inclusions. Moreover, most alloying elements or combinations of elements are effective only within certain ranges. Too little or too much of an alloying element or a combination of alloying elements may undesirably alter the microstructure and the properties of the connecting rod.
Alloying elements either can be admixed into a powder or can be prealloyed with an iron powder. Admixing of powders involves the mechanical mixing of two or more different powders to form a mixed powder having the individual grains of each of the initial powders. In contrast, prealloying involves the chemical addition of the alloying element with the iron powder to form a powder that has both iron and the alloying element in a single grain.
One element that can be alloyed with steel is copper. It is well known that copper can be a component in powder metal and can serve as a ferrite strengthener. A number of patents and patent applications disclose the use of copper as a separate admixed powder to be used in combination with ferrous powders. See U.S. Pat. No. 6,391,083 to Akagi et al., U.S. patent application Ser. Nos. 09/919,426 and 11/253,298 of Ilia. In particular, the Ilia patent applications disclose the use of the combinations of admixed iron powders and elemental copper powder in the production of connecting rods.
Likewise, the prealloying of copper along with other alloying elements in steel is known. U.S. Pat. No. 3,901,661 to Kondo et al. discloses a prealloyed steel powder for powder forging comprising up to 0.5 percent by weight carbon, 0.8 to 5.0 percent by weight copper, 0.1 to 0.7 percent by weight molybdenum, and up to 0.6 percent by weight manganese. The parts made from the powder in Kondo are quenched and tempered after powder forging to develop sufficient hardness (col. 7, lines 15-35). Moreover, molybdenum content in the disclosed range is a required prealloying element in order to attain sufficient hardness and strength (col. 5, lines 35-45).
Hence, it would be desirable to provide a more efficient process for making a connecting rod. In particular, it would be desirable to reduce the sintering time and temperature to process the connecting rod, thus reducing energy consumption, while still providing a connecting rod having sufficient or improved materials and mechanical properties.